At the present time, most cutting inserts comprise substrates made from cemented carbides because cemented carbides have a high degree of toughness and good wear resistance. However, the use of cutting tools and cutting inserts (“ceramic tools”) having ceramic substrates is increasing. These ceramic tools find use in a wide range of high-speed finishing operations and for the machining of difficult-to-machine materials at a high removal rate. The increase in use of ceramic tools may be due to improvements in alloyed ceramics and ceramic-matrix composites, as well as with the advances in ceramic processing technology. The ceramics used in cutting tools are typically inorganic, nonmetallic materials.
The production of ceramic tools typically involves the consolidation and sintering of powdered ceramics. Sintering provides the necessary densification of the consolidated powder and may optionally be performed under pressure. In pressureless sintering, the powder is first shaped into a green, or unsintered, body which may then be sintered to achieve the necessary densification. Hot pressing of ceramics involves heating along with simultaneous uniaxial pressing of the powder in a die. Although hot-pressed ceramics are more expensive, they may be prepared with a finer grain size, a higher density and higher transverse rupture strength than cold-pressed materials.
The ceramics currently used in cutting tools are based either on aluminum oxide (alumina, Al2O3) or silicon nitride (Si3N4). Other ceramics, such as, magnesia, yttria, zirconia, chromium oxide, and titanium carbide may be used as additives to aid sintering or to form alloyed ceramics with improved thermo-mechanical properties.
A ceramic tool comprising an aluminum oxide substrate may be used in metal machining with high cutting speed due to the chemical inertness and great strength of the aluminum oxide substrate. Commercially significant aluminum oxide ceramic tools were basically fine grained (less than 5 μm) Al2O3 with magnesia added as a sintering aid and grain growth inhibitor. Aluminum oxide ceramics may also be alloyed with suboxides or titanium or chromium to form solid solutions. The three main commercially available aluminum oxide based tool materials are Al2O3/Ti, Al2O3/ZrO2, and Al2O3 reinforced with silicon carbide (SiC) whiskers. Other Al2O3 base ceramics may have additives of TiN, TiB2, Ti(C,N), and Zr(C,N).
Alumina-zirconia (Al2O3/ZrO2) is an alloyed ceramic. The addition of zirconium oxide increases the higher fracture toughness and thermal shock resistance of an aluminum oxide substrate. The toughening of aluminum oxide with zirconium oxide exploits a specific crystallographic change, a martensitic type transformation, that results from an energy absorbing mechanism. The presence of metastable tetraorthoganol ZrO2 provides the potential for transformation under stress into a stable monoclinic structure. The transformation acts as a stress absorber and prevents, even when cracks exist, further cracking.
Typically, the zirconia oxide particles are concentrated at the aluminum oxide grain boundaries. Although fracture is intergranular, the presence of these particles is believed to provide additional toughness before failure can occur by fracture. The three most popular compositions contain 10, 25, and 40 weight % (wt. %) ZrO2 with the remainder being aluminum oxide. The 40 wt. % ZrO2 composition is close to the eutectic concentration. The higher ZrO2 compositions are less hard but tougher.
Cutting inserts may be coated to increase their resistance to wear. Single or multiple layers of coatings by chemical vapor deposition (“CVD”) or physical vapor deposition (“PVD”) may be applied to cutting tool. Titanium nitride (TiN), titanium carbon nitride (TiCN) and aluminum oxide (Al2O3) are among the most popular CVD coating materials for carbide-based cutting tools. Thin coatings (2 μm to 5 μm) on ceramic substrates have been developed primarily to limit chemical interactions between the tool and the work material and improve wear resistance. Examples of the very recent research efforts in applying the latest CVD coating technologies to ceramic cutting inserts include coated reinforced ceramic cutting tools, U.S. Pat. No. 6,447,896; coated silicon nitride ceramic cutting tools, United States Patent Application No. 2002/0076284; coated composite ceramic cutting inserts containing a hard phase dispersed with an alumina matrix phase, Published United States Patent Application No. 2002/0054794.
There is a need to develop new coating technologies for ceramic tools in order to further improve the wear and chemical resistance of ceramic tools to meet the ever-increasing demands on machining productivity.